“Pure” Graphene and “Graphenic Mixtures”
The term “graphene” refers, in the strictest sense, to the two-dimensional (one-atom-thickness) allotrope of carbon with a perfect planar honeycomb lattice of carbon atoms, a material which, when layered on itself hundreds or thousands of times forms graphite, and when rolled forms carbon nanotubes (“CNTs”), but which, until 2004, had never been isolated in the single flat sheet form that represents “pure” graphene. In 2004, however, two scientists at the University of Manchester in the United Kingdom, Novoselov and Geim, used micromechanical cleavage to successfully draw a single sheet of “pure” two-dimensional graphene from a block of graphite, a technique that, in light of the actual apparatus originally used to accomplish it, is generally referred to as the “scotch tape” method. See, e.g., “The Rise of Graphene,” Nature Nanomaterials 6 (2007): 183-191. This isolation of “pure” graphene for the first time was sufficiently revolutionary to result in the awarding of the Nobel in physics to these two scientists six years later, in 2010, as well as to spark a huge surge of research and development in the academic and industrial worlds into the unique properties of this material. See, e.g., See “Selling Graphene by the Ton,” Nature Nanomaterials 4(2009):612-614.
Although the term “graphene” in theory describes only the “pure” two-dimensional single-sheet honeycombed lattice of carbon atoms, in fact this term is regularly used more loosely to refer to what would more aptly be termed “graphenic mixtures.” Specifically, “graphene” is regularly used to denote not just “pure” graphene, but also 1) materials comprising multiple layers of “pure” graphene—in some examples 30 layers or more; 2) materials comprising a layer or layers containing more than carbon atoms, e.g., one or more layers modified facially (e.g., by epoxides), modified at the periphery by carboxyl groups, etc.; 3) materials comprising one or more layers that, because of facial and/or peripheral modifications (or for other reasons) contain defects in the arrangement of the carbon atoms, i.e., are not perfectly flat perfect honeycombs; 4) materials where there are non-carbon atoms in the carbonaceous layer; etc. Despite attempts to impose a standard nomenclature for what constitutes “graphene” (see, e.g., the terminology given by the International Union of Pure and Applied Chemistry, available at old.iupac.org/goldbook/G02683.pdf), at present the terminology is sufficiently diffuse that one recent article noted that, for example, Dow Chemical scientists are “still working through the process of exactly what it means to call something graphene.” See “Selling Graphene by the Ton,” Nature Nanomaterials 4(2009): 612-614.
Part of the problem with defining the meaning of “graphene” is that there is enormous commercial potential for materials that exhibit one or more of the hallmark properties of “pure” graphene, and such properties can manifest with mixtures of materials only some of which are “pure” graphene—i.e., in graphenic mixtures; therefore, both for sales and for marketing reasons such mixtures are frequently labeled “graphene” when they are not, at least not in the “pure” graphene sense. Specifically, “pure” graphene exhibits a number of remarkable physical properties, including: being the strongest material ever measured; having a thermal conductivity more than twice that of diamond; and, having an electron mobility among the highest of any semiconductor. See “Selling Graphene by the Ton,” Nature Nanomaterials 4(2009): 612-614. However these strength, thermal and electrical properties also manifest to a varying extent in materials that are mixtures of “pure” graphene with, e.g., “few-layer” graphene (that is, a number of stacked sheets of “pure” graphene), facially- or peripherally-modified graphene sheets, etc., and such “graphenic mixtures” are consequently valuable in their own right, even though their mixed nature may make them inappropriate for academic studies of “pure” graphene.
Powdered (Dry) Bulk Graphenic Mixtures Vs. Graphenic Dispersions and Slurries
Since graphenic mixtures have enormous commercial potential in their own right, a number of companies have developed industrial-scale methods to produce such mixtures in bulk powder form for incorporation into various end products. Thus for example Angstron Materials, Vorbeck Materials and XG Sciences have all developed various processes for fracturing graphite into graphenic mixtures with varying contents of “pure” graphene and, consequently, varying graphene-like properties, which they then supply in powdered (dry) bulkform for other purposes, as well as for their own in-house uses. See, e.g., “Selling Graphene by the Ton,” Nature Nanomaterials 4(2009): 612-614.
While such powdered (dry) bulk graphenic mixtures are appropriate for some applications, in many industrial processes it would be desirable to be able to supply such mixtures in particular dispersed or slurry forms in order to be compatible with the industrial processes using the particular graphenic mixtures to produce the final composition or product. Thus for example “pure” graphene consists of a honeycomb array of carbon atoms without any surface or peripheral chemical functionalities (i.e., a honeycomb of purely sp2 carbon atoms without epoxides, hydroxides, carboxyl groups etc.); however, without such functionalities to increase polarity, this “pure” graphene is highly insoluble in water and therefore is difficult to use in aqueous or other polar solvent systems. By the same token, functionalized graphene is soluble in water or other appropriate polar solvents, but is insoluble in non-polar solvent systems. Thus an industrial process taking place in an aqueous (polar) environment requires either functionalized graphene or graphene (or a graphenic mixture) in an appropriate solvent/surfactant/suspension mixture for compatibility with such an aqueous solvent. Similarly, industrial applications involving a non-polar solvent system requires a graphenic mixture solution designed for such a non-polar solvent system.
In light of the above, it would be highly advantageous to develop methods for the production of compatible graphenic mixtures where these mixtures are optimized not for the content of “pure” or approximately “pure” graphene, but rather for the exhibition of one or more of the hallmark strength, thermal or electrical properties of “pure” graphene while at the same time being optimized in terms of solubility or other properties necessary for compatibility with one or more end processes.